1. Field of the Invention
The present invention relates to an erroneous impingement protective device for a laser system. The device is arranged such that the operational length of the laser beam is limited to lie within a predetermined range to prevent the erroneous impingement of the beam caused by manipulating errors or operational faults. The generation of the laser beam is thus enabled only within a control range at a predetermined distance in the direction of the emitted beam from the end of the beam emitting handpiece.
2. Description of the Prior Art
A conventional protective device for a surgical laser is disclosed in "Lasers in Surgery and Medicine" Volume 1, Number 1, 1980. FIGS. 1 and 3 illustrate such a device. A laser beam emitting handpiece 1 is provided with a pair of light detectors 2,3 incorporated within pipes or tubes having fixed directive angles as shown. A visible beam 4 of axial laser light, such as from a helium-neon pilot source, is directed through the emitter end of the handpiece to a target 5, and the reflected and diffused light is sensed by the two light detectors 2,3.
FIG. 2 illustrates the output characteristics of the detectors for different targets; A.sub.0, A.sub.1 and A.sub.2 represent the distance-output characteristics of the light detector 2, and B.sub.0, B.sub.1 and B.sub.2 represent those of the light detector 3. Although the curves of these characteristics vary depending upon the reflection factors and inclinations of the target, there is substantially no change in the crossing points of the characteristic curves of the two detectors. Accordingly, the differential output of the two detectors lies at a substantially constant point even for different targets. By utilizing the distance corresponding to such crossover as a reference point, invisible surgical laser light, such as from a CO.sub.2 source, may be emitted or blocked whenever a predetermined distance range flanking such reference point is detected.
The conventional protective device having a pair of light detectors 2,3 is arranged, as shown in FIG. 3, such that a distance defining element 7, such as a differential comparator, serves to determine whether the differential value between the two detector outputs lies in the ON region or OFF region flanking the crossover point. A laser control unit 8 is adapted to control a laser power source 10 and surgical laser emitting unit 9 on the basis of the differential comparison, whereby the surgical laser beam is emitted only when the switch 6 on the handpiece is turned on by the operator and the target is present within the ON region. The pilot beam 4 always remains on, of course, when the system is in use; it is energized by a separate power source, not shown.
In order to make the operation of the system understood more clearly, flow charts for two types of conventional surgical laser systems are shown in FIG. 11 and FIG. 12. At step I in FIG. 11 a main switch is turned on, which starts the pilot laser oscillator emitting a beam as well as the monitoring of the distance between the handpiece and the target. After the power of the main laser beam and the emitting period are set at step III and the preparing switch for emitting the main laser beam is turned on at step IV, the main laser oscillator starts operating at step V. A hand or foot operated switch for emitting the main laser beam is next turned on at step VI, whereafter the distance between the handpiece and the target to which the pilot laser beam has been emitted is determined at step VII. If the distance is in the ON region a blocking shutter which has prevented the main laser from being emitted is withdrawn at step VIII and the surgery begins. After the surgical operation is finished, the switch for emitting the main laser beam is turned off at step IX, and the shutter is then closed to prevent the main laser beam from being emitted at step X.
Another sequence is shown in FIG. 12, wherein steps I to IV are identical to those in FIG. 11. After step IV the hand or foot switch for emitting the main laser beam is turned on at step V, and the distance between the handpiece and the target is then determined at step VI. If appropriate, the main laser oscillator starts and the main laser beam is emitted at step VII. After the surgical operation is finished the switch for emitting the main laser beam is turned off at step VIII, and the main laser oscillator stops working at step IX.
In the conventional system, the light detectors are oriented toward the reflected pilot beam from only one angle or direction on one side of the handpiece, whereas the handpiece may be aimed and manipulated relative to the living target or body from a number of different directions due to the irregular surface of the target, whereby the system tends to be unstable. Furthermore, a system of this type has the drawback that unstable operations will occur for targets having a higher than normal reflection factor since the structure of the system has been designed for intended use with diffuse reflections. As may be seen from FIGS. 4 and 5 showing the measurement of the reflection components from targets such as a ham and a beef liver, respectively, the ham surface produces a substantial amount of diffuse reflection components, while the beef surface produces more regular reflection components forming a sharp, high amplitude peak extending in one direction (at 90.degree. in this case). Thus, if only such sharp or regular reflection components are received by one of the two detectors, the resulting distance determination will be erroneous. Moreover, changes in the contour of the target surface when incised by the surgical laser will also lead to instabilities in the distance control operation.